Community structure in directed networks

We consider the problem of finding communities or modules in directed networks. The most common approach to this problem in the previous literature has been simply to ignore edge direction and apply methods developed for community discovery in undirected networks, but this approach discards potentially useful information contained in the edge directions. Here we show how the widely used benefit function known as modularity can be generalized in a principled fashion to incorporate the information contained in edge directions. This in turn allows us to find communities by maximizing the modularity over possible divisions of a network, which we do using an algorithm based on the eigenvectors of the corresponding modularity matrix. This method is shown to give demonstrably better results than previous methods on a variety of test networks, both real and computer-generated.Comment: 5 pages, 3 figure

Mixture models and exploratory analysis in networks

Networks are widely used in the biological, physical, and social sciences as a concise mathematical representation of the topology of systems of interacting components. Understanding the structure of these networks is one of the outstanding challenges in the study of complex systems. Here we describe a general technique for detecting structural features in large-scale network data which works by dividing the nodes of a network into classes such that the members of each class have similar patterns of connection to other nodes. Using the machinery of probabilistic mixture models and the expectation-maximization algorithm, we show that it is possible to detect, without prior knowledge of what we are looking for, a very broad range of types of structure in networks. We give a number of examples demonstrating how the method can be used to shed light on the properties of real-world networks, including social and information networks.Comment: 8 pages, 4 figures, two new examples in this version plus minor correction

Elastic precession of electronic spin states in interacting integer quantum Hall edge channels

We consider the effect of Coulomb interactions in the propagation of electrons, prepared in arbitrary spin states, on chiral edge channels in the integer quantum Hall regime. Electrons are injected and detected at the same energy at different locations of the Hall bar, which is modeled as a chiral Tomonaga-Luttinger liquid. The current is computed perturbatively in the tunneling amplitudes, within a non-crossing approximation using exact solutions of the interacting Green's functions. In the case of different channel velocities, the spin precession effect is evaluated, and the role of interaction parameters and wavevectors is discussed.Comment: 5 pages, 3 figure

Constructive role of non-adiabaticity for quantized charge pumping

We investigate a recently developed scheme for quantized charge pumping based on single-parameter modulation. The device was realized in an AlGaAl-GaAs gated nanowire. It has been shown theoretically that non-adiabaticity is fundamentally required to realize single-parameter pumping, while in previous multi-parameter pumping schemes it caused unwanted and less controllable currents. In this paper we demonstrate experimentally the constructive and destructive role of non-adiabaticity by analysing the pumping current over a broad frequency range.Comment: Presented at ICPS 2010, July 25 - 30, Seoul, Kore

Vertex similarity in networks

We consider methods for quantifying the similarity of vertices in networks. We propose a measure of similarity based on the concept that two vertices are similar if their immediate neighbors in the network are themselves similar. This leads to a self-consistent matrix formulation of similarity that can be evaluated iteratively using only a knowledge of the adjacency matrix of the network. We test our similarity measure on computer-generated networks for which the expected results are known, and on a number of real-world networks

Telangiectasia macularis eruptiva perstans: More Than Skin Deep

Systemic mastocytosis is a rare disease involving the infiltration and accumulation of active mast cells within any organ system. By far, the most common organ affected is the skin. Cutaneous manifestations of mastocytosis, including Urticaria Pigmentosa (UP), cutaneous mastocytoma or telangiectasia macularis eruptive perstans (TMEP), may indicate a more serious and potentially life-threatening underlying disease. The presence of either UP or TMEP in a patient with anaphylactic symptoms should suggest the likelihood of systemic mastocytosis, with the caveat that systemic complications are more likely to occur in patients with UP. TMEP can usually be identified by the typical morphology, but a skin biopsy is confirmative. In patients with elevated tryptase levels or those with frequent systemic manifestations, a bone marrow biopsy is essential in order to demonstrate mast cell infiltration. Further genetic testing for mutations of c-kit gene or the FIP1L1 gene may help with disease classification and/or therapeutic approaches. Rarely, TMEP has been described with malignancy, radiation therapy, and myeloproliferative disorders. A few familial cases have also been described. In this review, we discuss the clinical features, diagnosis and management of patients with TMEP. We also discuss the possible molecular pathogenesis and the role of genetics in disease classification and treatment

Large-scale structure of time evolving citation networks

In this paper we examine a number of methods for probing and understanding the large-scale structure of networks that evolve over time. We focus in particular on citation networks, networks of references between documents such as papers, patents, or court cases. We describe three different methods of analysis, one based on an expectation-maximization algorithm, one based on modularity optimization, and one based on eigenvector centrality. Using the network of citations between opinions of the United States Supreme Court as an example, we demonstrate how each of these methods can reveal significant structural divisions in the network, and how, ultimately, the combination of all three can help us develop a coherent overall picture of the network's shape.Comment: 10 pages, 6 figures; journal names for 4 references fixe

Generation of energy selective excitations in quantum Hall edge states

We operate an on-demand source of single electrons in high perpendicular magnetic fields up to 30T, corresponding to a filling factor below 1/3. The device extracts and emits single charges at a tunable energy from and to a two-dimensional electron gas, brought into well defined integer and fractional quantum Hall (QH) states. It can therefore be used for sensitive electrical transport studies, e.g. of excitations and relaxation processes in QH edge states

LOWER LEG MORPHOLOGY AND STRETCH-SHORTENING CYCLE PERFORMANCE IN YOUNG AND ELDERLY MALES

The purpose of this investigation was to examine bone and muscle characteristics of the lower leg and stretch-shortening cycle capabilities of the ankle in young (22.3 ± 1.3 yrs) and elderly (67.5 ± 3.3 yrs) males. Peripheral quantitiative computed tomography (pQCT) was utilized to assess bone stress-strain index, bone ultimate fracture load, muscle density, muscle cross-sectional area (CSA), fat CSA and muscle+bone CSA. Maximal voluntary isometric plantarflexion (MVIP) force and force-velocity measurments during a countermovement hop (CMH) and drop hops from 20, 30 and 40 cm (DH20, DH30, DH40) were also measured. Bone stress-strain index was significantly higher in young males as well as muscle density, muscle CSA and muscle+bone CSA in comparison to elderly males. MVIP peak force and rate of force development was significantly higher in young males in comparsion to elderly males as well. An analysis of the force-velocity curves indicated that young males had significanlty higher levels of force and velocity in both the eccentric and concentric phase during the CMH, DH20, DH30 and DH40 in comparsion to elderly males. The data from this investigation indicate that aging potentially negatively influences lower leg bone and muscle strength and this may be reflected in lower stretch-shortening cycle capabilities of the ankle